The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for a chemical mechanical polishing system.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar outer surface presents a problem for the integrated circuit manufacturer. If the outer surface of the substrate is non-planar, then a photoresist layer placed thereon is also non-planar. A photoresist layer is typically patterned by a photolithographic apparatus that focuses a light image onto the photoresist. If the outer surface of the substrate is sufficiently non-planar, then the maximum height difference between the peaks and valleys of the outer surface may exceed the depth of focus of the imaging apparatus, and it will be impossible to properly focus the light image onto the outer substrate surface.
It may be prohibitively expensive to design new photolithographic devices having an improved depth of focus. In addition, as the feature size used in integrated circuits becomes smaller, shorter wavelengths of light must be used, resulting in a further reduction of the available depth of focus. Therefore, there is a need to periodically planarize the substrate surface to provide a substantially planar layer surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted to a carrier or polishing head. The exposed surface of the substrate is then placed against a rotating polishing pad. The carrier provides a controllable load, i.e., pressure, on the substrate to press it against the polishing pad. In addition, the carrier may rotate to provide additional motion between the substrate and polishing pad. A polishing slurry, including an abrasive and at least one chemically-reactive agent, may be distributed over the polishing pad to provide an abrasive chemical solution at the interface between the pad and substrate.
A CMP process is fairly complex, and differs from simple wet sanding. In a CMP process, the reactive agent in the slurry reacts with the outer surface of the substrate to form reactive sites. The interaction of the polishing pad and abrasive particles with the reactive sites results in polishing.
An effective CMP process has a high polishing rate and generates a substrate surface which is finished (lacks small-scale roughness) and flat (lacks large-scale topography). The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad. Because inadequate flatness and finish can create defective substrates, the selection of a polishing pad and slurry combination is usually dictated by the required finish and flatness. Given these constraints, the polishing rate sets the maximum throughput of the polishing apparatus.
The polishing rate depends upon the force pressing the substrate against the pad. Specifically, the greater this force, the higher the polishing rate. If the carrier head applies a non-uniform load, i.e., if the carrier head applies more force to one region of the substrate than to another, then the high pressure regions will be polished faster than the low pressure regions. Therefore, a non-uniform load may result in non-uniform polishing of the substrate.
An additional consideration in the production of integrated circuits is process and product stability. To achieve a high yield, i.e., a low defect rate, each successive substrate should be polished under substantially similar conditions. Each substrate should be polished by approximately the same amount so that each integrated circuit is substantially identical.
In view of the foregoing, there is a need for a chemical mechanical polishing apparatus which optimizes polishing throughput while providing the desired flatness and finish. Specifically, the chemical mechanical polishing apparatus should have a carrier head which applies a substantially uniform load across the substrate.
In one aspect, the present invention is directed to a carrier head for a chemical mechanical polishing apparatus. The carrier head comprises a base, a support structure connected to the base by a flexure, and a flexible membrane connected to the support structure. The flexible membrane has a mounting surface for a substrate and extends beneath the support structure to define a chamber.
Implementations of the invention include the following. The flexure may be secured between an upper clamp and a lower clamp, and the membrane may be secured between the lower clamp and the support structure. The flexure may be substantially horizontal and annular, with an outer circumferential portion attached to the base and an inner circumferential portion attached to the support structure. The support structure may include an annular ring or a circular plate. A portion of the chamber above the plate may be connected by an aperture through the plate to a portion below. An outer edge of the support structure may have a downwardly projecting lip.
The carrier head may include one or more of the following: a housing connectable to a drive shaft to rotate therewith, a gimbal mechanism pivotally connecting the housing to the base to permit the base to pivot with respect to the housing, a retaining ring connected to the base and surrounding the flexible membrane, and a loading mechanism connecting the housing to the base to apply a downward pressure to the base. The housing may have a substantially vertical passage, and the gimbal mechanism may include a rod with its upper end slidable disposed in the passage. The gimbal mechanism may include a bearing base with a spherical outer surface connected to a lower end of the rod and a gimbal race with a spherical inner surface engaging the bearing base.
The support structure, flexure and membrane may be configured such that a downward pressure on the flexure is substantially balanced by an upward pressure on the support structure so that a downward pressure at the edge of the membrane is substantially the same as a downward pressure at other portions of the membrane. A surface area of the lower surface the support structure may. be approximately equal to a surface area of the upper surface of the flexure. An outer diameter of the clamp may be less than an outer diameter of the support structure.
There may be a gap between the support structure and the flexure, and there may be a passage through the support structure to carrying a fluid into the gap to force a slurry out of the gap.
In another aspect, to a carrier head includes a housing, a base, a loading mechanism, and a gimbal mechanism. The gimbal mechanism includes a rod having an upper end slidably disposed in the passage in the housing, and a slightly flexible member connecting a lower end of the rod to the base.
Implementations of the invention include the following. The member may be a ring with an inner circumferential portion connected to the rod and an outer circumferential portion connected to the base. The member may be bendable vertically but is rigid radially. A stop may be connected to the upper end of the rod to limit downward travel of the base.
In another aspect, a carrier head includes a housing, a base, a loading mechanism connecting the housing to the base to control the vertical position of the base relative to the housing, and a cushion attached to a lower surface of the housing to stop an upward motion of the base.
In another aspect, the carrier head includes a base, a first flexible membrane, and a second flexible membrane. The first membrane has a mounting surface for a substrate and defines a first chamber. The second membrane is connected to the base and positioned above the first membrane to define a second chamber. The second membrane is positioned to exert a downward pressure on the first membrane when fluid is forced into the second chamber.
Implementations of the invention include the following. The first membrane may be attached to a support structure which is connected to the base by a flexure. The second membrane may be positioned to contact either the support structure or the first membrane. A support structure may be connected to the base by a flexure, and the first membrane may be attached to and extend beneath the support structure to define the first chamber. The support structure may include a support ring, and the second membrane may be positioned to extend through the center of the support ring to contact the first membrane. The carrier head may be used in a polishing apparatus with a first fluid supply connected to the first chamber, a second fluid supply connected to the second passage, and a sensor for measuring a pressure in the second chamber.
In another aspect, the carrier head includes a base, a support structure connected to the base by a flexure, a first membrane portion, and a second membrane portion. The first membrane portion is connected to and extends beneath the base to define a first substantially circular chamber. The second membrane portion is connect to and extends beneath the support structure to define a second substantially annular chamber surrounding the first chamber.
Implementations of the invention include the following. A lower surface of the first membrane portion may contact or be attached to an upper surface of the second membrane portion.
In another aspect, the carrier head has a support structure having a bottom face, a flexible membrane defining a chamber, and a port for applying a vacuum to the chamber. There is a recessed region in the bottom face of the support structure. The membrane is arranged and configured to be pulled into the recessed region if the chamber is evacuated to produce a reduced pressure area between the flexible membrane and an upper surface of a substrate. The recessed region distributed in an asymmetrical fashion.
In another aspect, the invention is directed to a method of sensing the presence of a substrate in a carrier head. A first chamber, formed by a first flexible membrane having a mounting surface for the substrate, is pressurized. A second chamber formed by a second flexible membrane to a first pressure is also pressurized. The second membrane is positioned to contact the first membrane above the mounting surface. The second chamber is sealed. A substrate is placed against the mounting surface, and fluid is forced out of the first chamber to create a reduced pressure region to chuck the substrate to the mounting surface. Then the pressure in the second chamber is measured a second time.
Implementations include the following. If the second pressure is greater than the first pressure, then the substrate may be indicated as present. If the second pressure is equal to the first pressure, the substrate may be indicated as missing.
In another aspect, the invention is directed to a method of chucking a substrate to a mounting surface of a carrier head. A substrate is positioned against a mounting surface of a carrier head. Fluid is forced into a first chamber defined by a first flexible membrane to apply a downward pressure to an annular area of substrate, and fluid is forced out of a second chamber defined by a second membrane to pull the second membrane upwardly and create a reduced pressure region bounded by the annular area to chuck the substrate to the mounting surface.
Implantations of the invention include the following. The first membrane may contact either the substrate, a support structure, or the second membrane. The first chamber may include an annular volume.
Advantages of the invention include the following. The carrier head applies a uniform load to the substrate. The carrier head is able to vacuum-chuck the substrate to lift it off the polishing pad.
Additional advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention may be realized by means of the instrumentalities and combinations particularly pointed out in the claims.